The leukocyte differentiation antigen CD33 is a 364 amino acid transmembrane glycoprotein with sequence homology to members of the sialoadhesin family, including myelin-associated glycoprotein and CD22, as well as sialoadhesin itself (S. Peiper, 2002, Leucocyte Typing VII, White Cell Differentiation, Antigens, Proceedings of the Seventh International Workshop and Conference, Oxford University Press, p. 777).
Expression of CD33 appears to be highly specific to the hematopoietic compartment, with strong expression by myeloid precursor cells (S. Peiper, 2002). It is expressed by myeloid progenitor cells such as CFU-GEMM, CFU-GM, CFU-G and BFU-E, monocytes/macrophages, granulocyte precursors such as promyelocytes and myelocytes although with decreased expression upon maturation and differentiation, and mature granulocytes though with a low level of expression (S. Peiper, 2002).
In contrast, pluripotent hematopoietic stem cells that give rise to “blast colonies” in vitro (Leary, A. G. et al., 1987, Blood 69:953) and that induce hematopoietic long-term marrow cultures (Andrews R. G. et al., 1989, J. Exp. Med. 169:1721; Sutherland, H. J. et a., 1989, Blood 74:1563) appear to lack expression of CD33.
While the specific function of CD33 is unknown, its homology to sialoadhesin suggested a role in carbohydrate binding characteristic of the lectin family, a role later confirmed (S. Peiper, 2002).
Importantly, anti-CD33 monoclonal antibodies have shown that CD33 is expressed by clonogenic, acute myelogenous leukemia (AML) cells in greater than 80% of human cases (LaRussa, V. F. et al., 1992, Exp. Hematol. 20:442-448).
Due to the selective expression of CD33, immunoconjugates that combine cytoxic drugs with monoclonal antibodies that specifically recognize and bind CD33 have been proposed for use in selective targeting of AML cells. Such therapies are expected to leave stem cells and primitive hematopoietic progenitors unaffected. Immunoconjugates that utilize anti-CD33 antibodies include anti-CD33-ricin immunoconjugates that have been shown to be highly lethal to AML cells (Roy, D. C. et al., 1991, Blood 77:2404; Lambert, J. M. et al., 1991, Biochemistry 30:3234), yet spare the stem cells that support normal hematopoiesis and hematopoietic reconstitution (LaRussa, V. F. et al., 1992, Exp. Hematol. 20:442-448).
Additional studies using immunoconjugates have shown rapid targeting of radiolabeled anti-CD33 antibodies to leukemic blast cells in peripheral blood and marrow when administered i.v. (Scheinberg, D. A. et al., 1991, J. Clin. Oncol. 9: 478-490; Schwartz, M. A. et al., 1993, J. Clin. Oncol. 11:294-303). Rapid internalization of the antibody by the target cell was also observed in in vitro studies (Tanimot, M. et al., 1989, Leukemia 3: 339-348; Divgi, C. R. et al., 1989, Cancer Res. Suppl. Vol. 30: 404a). Evaluation of a humanized anti-CD33 antibody conjugated to the potent antitumor antibiotic calicheamicin (Gemtuzumab ozogamicin) in pre-clinical studies demonstrated specific killing of leukemia cells in HL-60 cell cultures, HL-60 tumor xenografts in mice, and marrow samples from AML patients (Hamann, P. R. et al., 2002, Bioconjugate Chem. 13: 47-58).
Based on the positive results of these pre-clinical studies, Gemtuzumab ozogamicin was evaluated in phase I and II clinical studies. In Phase I studies, the major toxicity observed was myelosuppression due to the expression of CD33 on myeloid progenitor cells (Sievers, E. L. et al. 1999, Blood 93: 3678-3684; Sievers E. L. et al., 2001, J. Clin. Oncol. 19: 3244-3254.). Phase II studies with a dose of 9 mg/m2 i.v. over 4 hours, repeated after 14 days, yielded a response rate of 30%. Marketing approval of Gemtuzumab ozogamicin was granted by the FDA in May 2000 with indication for the treatment of patients with CD33 positive AML in first relapse who are 60 years of age or older and who are not considered candidates for cytotoxic chemotherapy. Post-marketing reports have indicated the potential for significant toxicity, especially venoocclusive disease (VOD), which has led to labeling revisions and initiation of a patient surveillance program. Much of this toxicity may be related to the drug component calicheamicin, which was shown to cause hepatotoxicity in pre-clinical models, and therefore may not be a direct result of targeting CD33.
While the results discussed above suggest that immunoconjugates comprising an anti-CD33 antibody and a cytotoxic drug may be successfully used in the treatment of AML, there is a need for immunoconjugates that are both safe and effective. The present invention is directed to these and other important ends.